Evolved Melt Migration in Primitive Lower Crustal Gabbros at IODP Site U1309, 30N, Mid- Atlantic Ridge

IODP Hole U1309D (Legs 304/305) penetrated 1415 m into the core-complex of the Atlantis Massif at 30°N, Mid-Atlantic Ridge. Of the 75% recovered rock, 96% is gabbroic, comprising olivine-rich troctolite, (olivine-) gabbros, (olivine-) gabbronorite and evolved ferrogabbro. Possibly, a larger-scale magmatic cycle is preserved between 600 und 1240 mbsf, as marked by olivine-rich troctolites at the base and more common gabbronorites near the top. In detail, however, internal magmatic contacts are numerous with more evolved gabbros intruding into less evolved ones. Here we present a microprobe overview of the 800-1200 mbsf interval and a detailed study of the transition from evolved rocks at 1240 mbsf to the overlying primitive gabbros. Geochemically, there is a good correlation between the REE in cpx, the Mg# in cpx, and lithological evolution. The data can be modeled as simple incremental batch fractionation from 5 to >80% of a primitive average N- MORB. The need for replenishment of the melt body seems not present. In the more evolved gabbros, the fractionation of apatite is indicated and observed. The rims of nearly all clinopyroxene grains were overprinted by an evolved melt. A three-stage model is required to explain the data set: (1) a cumulus phase; (2) displacement of residual melt bodies and formation of new cumulate bodies. These two processes can explain the presence of well-equilibrated, but differently evolved cores of clinopyroxene. (3) Compaction of a residual melt and differentation of it as it migrated through the cumulus pile. This stage explains the late overprint of the rims of clinopyroxene. In a following cycle, a new pluton would dock to the base of the earlier body. We are still somewhat uncomfortable with what is a rather abrupt transition between the olivine rich troctolites (containing high Cr in clinopyroxene and high Ni in olivine) and the overlying olivine gabbros and the large volume of melt (without replenishment) required in the model.